Introduction

Although the wastewater treatment systems are used for improving the quality of wastewater, energy consumption, greenhouse gas emissions, the use of chemicals, and some toxic emissions cause adverse impacts such as noise, odor, and sludge. However, the potential environmental impacts associated with wastewater treatment (WWT) systems are not considered. Recently, the WWT has improved in developing countries, but potential environmental impacts remain a significant challenge. Analyzing these impacts can help decision-makers choose the best treatment option, or modify existing systems to reduce the potential environmental impact of their activities. The life cycle assessment (LCA) method is the one that is most frequently used to assess a WWTP’s environmental impacts. This method stands out because it considers resource and energy consumption, air emissions, and waste creation in order to evaluate a WWTP beyond the trade-off between process efficiency and end effluent quality (Lopes et al. 2020). The first LCA for WWTs was published in 1997 (Roeleveld et al. 1997). Today, this method analyzes various environmental consequences related to all stages of a product, service, or process (Odum and Nilsson 1997; Buyukkamaci 2013). LCA has many strengths, including evaluating the material and energy efficiency of a system, solving environmental problems without creating another problem through pollution transmission, and providing a standard for improvement. The LCA’s superiority over other methods is due to its unique and comprehensive approach, strategic environmental assessment, cost–benefit analysis, material flow analysis, environmental assessment, or ecological footprint (Finnveden et al. 2009; Chen et al. 2012). In addition, a top-down comparison of different scenarios of wastewater and waste management systems in order to assess environmental impacts is done by this method (Zarea et al. 2019). (Resende et al. 2019) indicated that greenhouse gases produced from the septic tank and nutrients in the effluent have potential impacts on climate change, eutrophication, and photochemical oxidants. In addition, electricity consumption accounts for only 7% of the total impacts of climate change. Further, the exploitation stage has the most tremendous potential for environmental impacts by using open LCA software based on the environmental and economic performance of two small-scale decentralized WWT systems, along with constructed wetlands. Measured the total energy and GHG footprint from wastewater infrastructure, including energy consumption and emissions from the transmission and showed a two-way relationship between energy storage and environmental consequences and greenhouse gas emissions. Evaluated two energy-saving systems (wetland constructed with slow infiltration) and a conventional system for small and decentralized communities by indicating that the impact of energy storage in wastewater systems on the environment, especially the global warming index, is negligible. Comparing conventional WWTP systems with natural wastewater treatment systems such as hybrid wetland and high-speed algal pond systems showed that nature-based solutions are environmentally friendly options. However, the ordinary WWTP offers the weakest performance due to the high consumption of electricity and chemicals. Limphitakphong et al. 2016 studied the analysis of the environmental and economic performance of waste and wastewater management in Aarhus, Denmark, and the results showed that resource and economic efficiency, and collection and separate sludge transfer to biogas units are considered as the most sustainable solution. The results of a study of the carbon footprint of seven WWTPs with different technologies in Denmark and Sweden indicated that direct emission of greenhouse gases is the most critical factor in carbon footprint by using the life cycle assessment approach (Delre et al. 2019) focused on the life cycle of a WWTP unit in Brazil. They suggested that there is a two-way relationship between the use of materials and energy in the construction phase and between low energy consumption and materials in the operation phase in these systems, where the treatment process consisted of an up-flow anaerobic sludge blanket reactor followed by artificial wetlands. Gallego-Schmid and Tarpani 2019 conducted a comprehensive review of the main challenges and shortcomings identified in 43 LCA studies of wastewater treatment plants in developing countries. This study considered the main obstacles using LCA in developing countries such as the lack of specific databases, data transparency, exact life cycle index (LCI), and knowledge and interest in using LCA. The study of (Tabesh et al. 2019) about the important sources of environmental impact on WWTP Tehran by the LCA method showed that using biogas instead of natural gas plays a significant role in reducing the environmental impacts of WWTP Tehran.

Iran always had problems with the lack of a proper WWTP infrastructure. Only about 40% of domestic wastewater is wholly treated (Tabesh et al. 2019). The benefits of the LCA method are not correctly recognized, and the environmental approach to infrastructure construction is not adequately considered by policymakers and decision-makers. Thus, this method has been less used in solving the real problems of urban water and sewage, and most of the studies have been conducted in the field of waste management (Feizi Masoule and Tabesh 2013). Based on (Gallego-Schmid and Tarpani 2019), LCA research in wastewater is mainly spread in developed countries. Therefore, LCA studies for WWTPs in developing countries can be considered completely new and necessary. A sustainable approach to wastewater treatment strengthens the planning, design, operation, maintenance and management of treatment equipment which reduces the use of non-renewable resources, decreases environmental impact, and is economically and socially acceptable. The sustainability trend in the wastewater industry provides an opportunity for managers to rethink equipment management practices and use new methods for greater flexibility and improved performance.

The aims of present study are to analyze a wastewater treatment and reuse project in Iran (Ahvaz) as a developing country to clarify the benefits and impacts of the treatment plant and wastewater reuse by using the LCA method through combining process-based LCA and input–output LCA in one framework. Therefore, in this research, the existing challenges and shortcomings should be eliminated as much as possible. Furthermore, the role of two different scenarios (the first scenario represents the current state of the plant (discharge the treated wastewater to the river of Karun), while the second considers reusing treated effluent in farms. reusing treated effluent in farms) in greenhouse gas emissions, discharge to biological resources was compared.

Materials and methods

A brief description of Wastewater treatment systems

Ahvaz is located at an elevation of 12 m above sea level with a population of 1,303,000 people and an area of 185 square kilometers (Zahedi et al. 2018). The location of the city in Khuzestan plain has turned it into an area with almost flat topography. Climatically, the city of Ahvaz can be classified as a warm and semi-arid area, which is characterized by scorching summers and mild winters. The biological treatment plant of activated sludge, with an area of 12 hectares in the west of the city and adjacent to the largest river in Iran, Karun, and has been working since 2001 to treat part of the domestic sewage of Ahvaz city. Three modules were planned for implementation. Currently, only one module with an average capacity of 40,800 cubic meters per day and a maximum capacity of 60,000 cubic meters during floods is in operation (Fig. 1) The following are the key components of Ahvaz’s WWTP: pumping station, bar screen, grit chamber, primary clarifier, aeration lagoons, secondary clarifier, sludge treatment (anaerobic digestion), gas storage tank, and sludge drying bed (AWC 2019) (Fig. 2).

Fig.1
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Location of Ahvaz city in Iran and location of the treatment plant in Ahvaz city

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Fig.2
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Flow diagram of West Ahvaz Wastewater Treatment Plant

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Life cycle assessment (LCA)

The LCA method is used to assess the environmental impacts. According to LCA standards in the ISOFootnote 114000 series, this method has four main stages: goal and scope definition, the life cycle inventory (LCI) analysis phase, life cycle impact assessment (LCIA), and the interpretation of results (Fig. 3 (ISO14000)).

Fig. 3
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The different stages of life cycle assessment (IS0 14040, 2006)

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Goal and scope definition

In this study, the impact of the life cycle of the West Ahvaz Wastewater Treatment Plant was evaluated to estimate the environmental performance of the complex. In addition, some solutions were proposed to reduce the negative impacts using different scenarios. In the first scenario, the current state of the treatment plant and the discharge of treated wastewater into the Karun River were explained. In the second scenario, the treated wastewater is used after leaving the treatment plant to irrigate farms. The impact was evaluated by using two methods in CML2001 and Eco-Indicator99 in SimaPro®9.0.0 software. The reasons for choosing CML2001 and EcoIndicator99 as methods for Life Cycle Assessment (LCA) are that they are widely recognized and used, comprehensive in assessing a wide range of environmental impacts, have well-established databases that provide necessary data for the assessment, and have specific focuses that may align with the goals of the assessment. Additionally, these methods enable the comparison of results with other studies, and the findings can be more easily communicated and understood by stakeholders (Lopes et al. 2020). The functional unit is used to provide a reference for communicating between inputs and outputs to ensure that the results are comparable, and it was selected as “one cubic meter of treated effluent.” In this treatment plant, the final sludge after digestion and dewatering is used as fertilizer in the green space of the treatment plant and its surroundings. Therefore, energy transportation is insignificant. The treated wastewater was also discharged into the river. However, using sludge as a fertilizer due to its nitrogen and phosphorus leading to savings in the consumption of these substances, and discharging the treated effluent into the river saves water, they can have destructive impacts on soil and surface water sources due to the presence of heavy metals. Table 1 shows the savings of WWTP basic materials in Ahvaz. Although the produced gas in the digestion used to be collected, it now enters the air directly due to the poor performance of the digestion (AWC 2019).

Table 1 The amount of daily savings for important materials in the WWTP (AWC 2019)
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System boundaries

Different system boundaries were selected for the LCA of WWTP systems. Most of the studies consider the decommissioning phase of WWTP systems and ignore the construction phase. Based on the studies related to the life cycle of the wastewater treatment plant, the operation phase has significant impacts compared to construction and end-of-life phases (Tabesh et al. 2019; Limphitakphong et al. 2016; Lopes et al. 2020; Garfí et al. 2017; Resende et al. 2019). The boundaries of the system expanded to include the sludge disposal phase as well as the use of treated sludge and effluent in green space fertilization and field irrigation, as shown in Fig. 4.

Fig.4
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Diagram of System boundaries and inputs and outputs studied

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Life cycle inventory: LCI

The quantification process involves collecting data and computational procedures to quantify energy and raw material consumption, atmospheric emissions, release into the water, solid waste, and other materials released throughout the product life cycle. Life cycle inventory analysis creates a list including the amounts of pollutants released into the environment and the amount of energy and consumed materials. The results can be separated by life cycle stages, host environment (air, water and land), special processes or any other combination (Guinée and Lindeijer 2002).

Data quality requirements

Based upon the ISO 14040 (2006), data quality requirements are required in order to prove the reliability of the study results and perform the correct interpretation of the LCA. The data represent the actual scale of system utilization construction and operation, collected from the WWTP project (Guinée and Lindeijer 2002). The laboratory analyses of the raw wastewater and effluent contributed to the representation, consistency, and completeness of the study. For the LCI phase, the data were obtained from various sources, including Ahwaz Water and Wastewater Company, interviews with experts in the West WWTP of Ahwaz, field visits, tests on sewage and production sludge for the LCI phase. Thus, SPSS software was used to evaluate the data related to eight years of WWTP in West Ahvaz (2009–2017), including the amount of: ammonia, nitrite, nitrate, total nitrogen, phosphate, sulfate, total solids, organic nitrogen, biological oxygen demand (BOD), and chemical oxygen demand (COD). Experiments included the measurement of chemical oxygen demand (COD), total organic carbon (TOC) at different stages of WWT and measurement of heavy metals in wastewater inlet and outlet of the treatment plant. In addition, the heavy metals in wastewater and sludge were determined using an ICP-OES device. Due to the limited information about soil properties and groundwater level, the impacts of field irrigation with treated wastewater on soil salinity, the transfer of heavy metals from sludge and irrigation water to groundwater were not considered. Furthermore, the most negative condition of sludge spreading in the green space, in which all the heavy metals transfer from the sludge to the environment, was considered. Therefore, more research is needed to determine the exact amount of heavy metal uptake by plants, as well as the amount transferred to another stage as leachate.

In addition, direct atmospheric CH4 and N2O emissions were calculated based on the Intergovernmental Panel on Climate Change Reporting Algorithms (IPCC) (Hiraishi et al. 2014; Metcalf et al. 1991). Table 2 shows the amounts of gases emitted at different stages of treatment. The total energy used to perform various processes such as pumping, activated sludge, and nitrification in the studied treatment plant is about 650 kW, which is supplied from the municipal electricity network. Since energy impacts which are related to the municipal electricity network are taken into account in the capacity of the power plant, they have not been considered in the present project. No disinfection or chlorination process was conducted to preventing damage to the aquatic animals in the Karun. Therefore, no chemicals were used in this treatment plant during WWT, and all processes were completely biological.

Table 2 Exhaust gases from sludge different process
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Impact assessment life cycle

According to the ISO standard the life cycle impact assessment (LCIA) is a life cycle assessment study that includes mandatory elements (classification and description) and voluntary elements (normalization and weighting).

There are different methods for LCIA. According to (Jolliet et al. 2003), life cycle assessment methods are divided into two main groups. In the midpoint method, modeling is stopped before the end of the work and the results of the catalog analysis are related to the middle groups. For each impact group, a reference index is assigned and the data related to the reference equivalent are converted. EDIP97, CML2001 and LUCAS are among the midpoint methods (Sala et al. 2012). In the endpoint method, the impacts group corresponds to the last step of the impacts path. In general, the final indicators are divided into human health, environmental health, and availability of resources (Bare and Gloria 2008). (Reap et al. 2008) EcoIndicator99 and Lime can be mentioned among the endpoint methods (Sala et al. 2012). Reap et al. (2008) reported that endpoint impact categories are more incomplete than midpoint and its uncertainty is more in comparison with the other group impacts. In contrast, the interpretation of midpoint impact categories is more difficult because they are not directly related to the scope of protection.

To cover the work categories types, the impact was assessed by using two methods including CML2001 from the midpoint group and Eco-Indicator99 from the endpoint group in SimaPro software. (Gallego-Schmid and Tarpani 2019) indicated that the CML impact evaluation method is considered as one of the most valid and established methods for performance estimation and is the most widely used WCTP LCA in developing countries. Therefore, CML can be considered a valuable LCA method. Eco-Indicator 99 is also one of the most widely used methods in LCA. Based on this method the impact was evaluated in two steps: (a) modeling the actual damage, (b) normalization and weighting (Tajrishy 2010). The advantage of these two methods is that they are available in SimaPro. SimaPro can effectively create and analyze LCA models and offer many analysis options to experts and decision-makers. In addition, SimaPro software can access life cycle inventory data at all stages of modeling and analysis (Audenaert et al. 2012). Thus, it is one of the most useful software for modeling many life cycle impacts and environmental performance.

Normalization

Normalization makes it possible to compare all environmental impacts on the same scale. Normalization of the impact category indicator results was done using the World, 1995 criteria in CML2001 method and European values in EcoIndictor99. Due to the most important environmental issues in the study area, impact categories in these methods were examined.

Result and discussion

In the last step, results interpretation, statistical analysis of data using SPSS software and the results of the experiments performed were presented in the form of graphs and tables. Further, the potential environmental impacts of WWT in the Ahwaz WWTP unit were calculated and some solutions were proposed in order to improve environmental sensitive points based on the study results.

Table 3 illustrates the characteristics of treated wastewater and the result of statistical analysis of the data. Figures 5 and 6 compare the quantity of heavy metals (the results of measurement tests) in the sludge and effluent with the different Iranian and American (EPA) standards. According to the standard of Iran Environmental Protection Organization the amounts of heavy metals in the treatment plant sludge were standard, except zinc. However, based on the Limits of Heavy Metals to discharge in surface water, the amounts of cadmium, lead, and zinc more than the allowable limit. Furthermore, the amounts of cadmium, lead, copper, zinc, and cobalt exceeded the Limits of Heavy Metals for Agricultural and Irrigation Purposes in Iran. On the other hand, all heavy metals in the effluent, except nickel, exceeded the standards in Iran and EPA (Fig. 6), so some solutions will be presented.

Table 3 The characteristics of treated wastewater
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Fig. 5
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Comparison of heavy metals in sludge with standards

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Fig. 6
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Comparison of heavy metals in effluent with standards

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Based on the ISO 14040 (2006), the interpretation includes: (a) recognize considerable matters arising from the outcomes of the LCI and LCIA phases of an LCA, (b) assessment the study regarding the sensitivity, completeness, and compatibility; and (c) presenting conclusions, limits, and recommendations. The interpretation of the CML2001 and EcoIndictor99 methods is presented as follows.

Life cycle impact assessment (first scenario)

Life cycle impact assessment method CML2001

Figure 7 shows the potential impact of LCA per WWT unit using the CML2001 method, which indicated the highest impact potential for human toxicity with a value of 4.29 × 1013 and global warming with a value of 3.67 × 1013 due to the presence of heavy metals in the treated effluent and dewatered digested sludge, respectively. Furthermore, the results indicated large volumes of methane released during anaerobic digestion, which are in line with those of the study of (Alanbari et al. 2015), which focused on the most effective category of work in WWTP Karbala, Iraq by using SimaPro7.0 global warming. As shown in Fig. 8, in the human toxicity impact category, the figure for hydrogen sulfide had the most significant impact, with a percentage of 74%, in the air sector. The antimony (with 83%) in the water sector, and the chromium (with 43%) in the soil sector, ranked first. In general, the toxic impact of heavy metals in water was much greater than the other two sectors. The results show the treatment process was not effective in reducing and removing heavy metals; therefore, adding chemical and advanced treatment was proposed in this treatment unit. As shown in Fig. 7, these elements in the treated effluent and dewatered digested sludge were responsible for the negative impacts (with 100%) on land toxicity, freshwater toxicity, and human toxicity. The concentration of heavy metals in fertilizers and effluent depends on the concentration of heavy metals in raw wastewater. Hence, controlling the source of input (raw wastewater) is the best solution. Figure 9 shows the direct and indirect emissions of greenhouse gases and compares the contribution of each stage of the WWT to the global warming impact. According to this figure, CH4 with 0.577 (65%) had the largest share in global warming, and anaerobic digestion of sludge in digestions with 0.3 (52%) was the largest producer of this destructive gas. Treatment process units in WWTP, sludge digestions, and sludge receiving environment emit these emissions directly. It is claimed that there is a great deal of uncertainty in determining the amount of N2O emissions from biological nutrient removal processes. Foley et al. (2010) suggested that emission factors in N2O are lower in units with higher levels of nitrogen removal than in units with medium levels of nitrogen removal (Lopes et al. 2020). Biogas production is one of the most important characteristics of anaerobic reactors, which is mainly composed of CH4. Biogas production plays a pivotal role in two-stage anaerobic digestion reactors since it can be positive if recycled or negative if sent directly to the atmosphere. Unfortunately, biogas is sent to the atmosphere without any treatment in most WWTP networks built in developing countries (Lopes et al. 2020). Although in the past biogas was collected in special tanks and it provided the required energy for digesting the sludge and WWTP, now the biogas enters the atmosphere directly due to damaged digestions. Therefore, this problem should be given special attention and the practical solutions for the use of biogas should be found in order to increase the environmental productivity of this WWTP. However, how to use biogas energy in small WWTP without increasing operating costs is regarded as one of the major challenges in principled and economic implementation of circular digestion (Lopes et al. 2020). Direct emission from the operation stage of the WWTP is 0.885 kg/m3.eq (Fig. 8), which is slightly higher than what was mentioned in the literature (for example, 0.6 kg/m3.eq in (Foley et al. 2010)). The production of CH4 in the anaerobic process mainly depends on the amount of degradable organic matter in the wastewater inlet and temperature, and temperature increases the production of CH4 (Hiraishi et al. 2014). Two-stage anaerobic digestion reactors have the greatest contribution to the emission of this gas, provided that CH4 is not collected or ignited. According to (Bressani-Ribeiro et al. 2019), the use of thermal energy for biogas recovery (2000 ≥ PE) is a good alternative to reduce the environmental impacts of using anaerobic digestion reactors and WWTP on a small scale.

Fig.7
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Description of the WWTP impacts with the CML2001 impact assessment method. Eu = Eutrophication, G.W: Global Warming, Acid = Acidification, T.E = Terrestrial ecotoxicity, F.W.E = Fresh water aquatic ecotoxicity, H.T = Human Toxicity

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Fig. 8
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The contribution of each metal and emission in different environments in Human Toxicity

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Fig. 9
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The contribution of each wastewater treatment unit in Global Warming

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Life cycle impact assessment method eco-indicator99

Figure 10 shows the impact potential of each step of the WWT using the Eco-indicator99. In this method, different categories of impacts are examined. Further, the result of each impact category is normalized on the same scale to compare all environmental impacts. Based on the results, the “Ecotoxicity” with a value of 5.2 × 10–3 and “Carcinogens” with a value of 2.28 × 10–3 had the most negative impact on the environment due to the high amounts of heavy metals in the treated wastewater and dewatered digested sludge. Although using the treated sludge as a fertilizer saves the mineral fertilizers, it has pivotal impact potential in human toxicity and ecological toxicity impact categories. Therefore, it is strictly necessary to control the heavy metals in the sludge seriously. Reducing heavy metals in the wastewater sludge treatment process can significantly decrease the negative impacts of sludge use. Those mineral fertilizers also contain significant amounts of heavy metals. Incineration is one of the ways to reduce the negative impacts of landfills or the use of sludge as a fertilizer. However, this process itself releases toxic gases into the environment and requires energy consumption. Comparing the life cycle environmental burdens of sludge use scenarios in agriculture, incineration, and landfill can help decision-makers select the appropriate process. For example, (Xu et al. 2014) analyzed the life cycle of environmental and economic burdens in various scenarios of sewage sludge treatment (anaerobic digestion, dewatering, incineration, landfilling, and agricultural use technologies) in China. The results showed that waste incineration and disposal technologies had the lowest and highest environmental impacts, respectively. Furthermore, completely anaerobic digestion is the best option to reduce environmental and economic loads. In the latest UK study, performed life cycle environmental impacts for five sludge management options (i.e., (i) use the anaerobically digested sludge in agricultural; (ii) use the composted sludge in agricultural; (iii) incineration; (iv) pyrolysis; and (v) wet air oxidation), the use the anaerobically digested sludge with the recovery of nutrients and electricity had the lowest environmental impacts (Tarpani et al. 2020). However, use the anaerobically digested sludge in agriculture had the highest freshwater ecotoxicity owing to heavy metals. Hence, it is necessary that heavy metals in the sludge strictly be controlled for this alternative to reduce freshwater ecotoxicity in comparison with the thermal processes.

Fig. 10
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Description of the WWTP impacts with the Eco-indicator99 impact assessment method. Car = Carcinogens, Acid/Eu = Acidification/Eutrophication, C.Ch = Climate Change, Eco = Ecotoxicity, R.I = Respiratory Inorganics

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Life cycle impact assessment (Second scenario)

Presently, the treated wastewater of Ahwaz is discharged into the Karun River. Therefore, it has destructive impacts on surface water resources. The second scenario is defined to store water and study the environmental impacts of using treated wastewater to irrigate fields, including the use of Ahwaz WWTP treated wastewater to irrigate farms. The results are compared with the first scenario (i.e., discharge in the Karun River). Since the Eco-indicator 99 method does not consider the impacts of nutrients and acids discharge into the water and soil (Frischknecht et al. 2007), only CML2001 was used to compare the two scenarios.

Table 4 shows the normalized results of the two scenarios. Eutrophication potential due to treated wastewater was 0.046 kg/m3 equivalent to phosphate. Eutrophication plays a crucial role in WWTP life cycle assessment studies since it can describe the balance between process efficiency and effluent quality (Hellström et al. 2000). Lack of discharging treated wastewater into water areas, increasing the degree of advanced purification to remove nitrogen and phosphorus, and reducing wastewater nutrients are three solutions for reducing the negative impact in the first scenario. It is worth noting that the consumption of materials and energy increases while increasing the degree of purification, leading to an increase in negative impacts in other groups. As shown in Table 4, irrigation of farmland added to the previous impact categories using treated wastewater completely can eliminate the eutrophication impact category. Thus, the terrestrial ecotoxicity impact group is only related to the heavy metals in the treated wastewater (Table 4), which is negligible compared to the amount of drought toxicity due to heavy metals sludge. Eutrophication is one of the basic criteria for determining the sustainable treatment of wastewater. The results are consistent with those of (Tabesh et al. 2019)’s study. (Miller-Robbie et al. 2017) found that emissions decreased by up to 33% throughout the life cycle system by comparing the effect of WWT-induced emissions by reusing untreated agricultural effluent at surface flows using the LCA method. Thus, discharging the treated wastewater to surface water sources has a significant (eutrophication, acidification, terrestrial ecotoxicity and so on) negative impact on the quality of these sources, which should be eliminated.

Table 4 A comparison of impacts associated with using effluent for irrigation and discharging it into the river
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Conclusion

The aim of present study is to evaluate the potential environmental impacts using the life cycle assessment method in the Ahwaz WWT system based on CML2001 and Eco-indicator99 methods. The most effective parts of the system and its inputs were identified on each impact category in the whole environment. Thus, the environmental impacts of using treated wastewater in agricultural irrigation were assessed and compared with the impacts of discharging treated wastewater into the river. The results obtained from the CML2001 database indicated that human toxicity and global warming are the most important impact categories, in which the heavy metals, especially antimony in the effluent and chromium in treated sludge, and methane were the most effective factors. The results of the Eco-indicator99 database indicated that ecological toxicity and carcinogenicity are the most effective impact categories. The heavy metals in the effluent which was discharged into the river and the sludge which was used as fertilizer played important roles in creating these categories. The results of the alternative scenario suggested that reusing treated wastewater in the irrigation sector could drastically reduce the destructive impacts on the environment. In addition, it can reduce the release of nutrient pollutants into the aquatic environment after resource recovery. In other words, it eliminates the eutrophication impact category, so it would definitely be better option. However, the scenario of discharging treated wastewater into the river has the worst impact on marine eutrophication and human health. In general, the results highlight the importance of profound control over the amounts of heavy metals in wastewater. Therefore, special attention should be paid to heavy metals to ensure that poisoning with effluent and sludge from sewage used for agricultural purposes cannot exceed the standard. The water and wastewater Company should invest in programs which prevent from entering heavy metal sources into municipal wastewater and seek to remove heavy metals and improve the quality of treated wastewater effectively. Further, the important impact of global warming should be highlighted. Two-stage anaerobic digestion can reduce the impacts of global warming since CH4 is the most important air pollutant among emissions. Emissions (e.g., CH4 and N2O) have a significant impact on the WWTP operation phase. Therefore, WWT experts should not ignore air pollutants in evaluating the impacts of wastewater treatment, especially in the use of anaerobic processes for some treatment stages such as sludge digestion by emphasizing the need to reduce direct emissions. This type of information is necessary for water and wastewater companies to understand the environmental performance of their services better. Finally, the main challenges of LCA studies in developing countries i.e., the lack of complete and effective access to the required data, low quality of the data, and lack of native and regional databases, must be considered.